Biochemical defence mechanisms

Cytokines are peptides that are produced and secreted by cells of the immune system. They organise the immune response to invasion by a pathogen by communicating between the different cells. They are synthesised in the immune cells as precursor proteins (pro-proteins) from which a peptide is removed by a proteolytic enzyme to produce the active cytokine, prior to secretion. This enzyme is a serine protease. Perhaps surprisingly, some viruses are capable of synthesising serpins which inhibit this enzyme in the immune cells, so that secretion does not occur and communication and integration of the immune response to the viral infection is lost. This is one of many biochemical mechanisms by which pathogens can reduce or overcome the defence mechanisms of the host (Chapter 17). 

The presence of gene amplification in insects, although so far investigated only in mosquitoes and aphids, emphasizes the importance of this genetic process as a survival mechanism under high insecticidal selection pressure. The biochemical cost to the insect of such amplification of a defence mechanism may be considerable. 

Vericel, E., Narce, M., Ulmann, L., Poisson, J. P., and Lagarde, M. Age-related changes in antioxidant defence mechanisms and peroxidation in isolated hepatocytes from spontaneously hypertensive and normotensive rats. Mol Cell Biochem 132 (1994) 25-29. 

The third section of the periodic table comprises transition metals or the d-group elements, which are involved in many biological defence mechanisms against toxic radicals. As in chemical catalysis the same metals are active in biochemical systems. Surprisingly these redox mediators are involved in both, the generation and the removal of reactive oxygen species. 

Various mechanisms have been developed by plants to alleviate stress imposed by exposure to toxic metal concentrations. Although the biochemical and molecular bases of adaptation and resistance of plant communities to high metal levels will be discussed extensively in Chapter 3, a survey summarising the possible anti-metal stress defence strategies and mechanisms forms an essential part of this chapter on toxicology. Every plant species, whether it is resistant or not to high metal concentrations, possesses defence responses against increased metal concentrations in the cell indeed. 

Levonen AL, Landar A, Ramachandran A, Ceaser EK, Dickinson DA, Zanoni G, Morrow JD, Darley-Usmar VM (2004) Cellular mechanisms of redox cell signalling role of cysteine modification in controlling antioxidant defences in response to electrophilic lipid oxidation products. Biochem J 378(Pt 2) 373-382... 

---